A new Legends and Lore book, without god stats. A big book about the various gods, religions, legends, and beliefs, focusing on the priesthoods and the followers. The 3 religion books from 2nd edition were fantastic for this, and a 3rd edition one, with details on various earthly gods (Thor, etc), as well as the D&D/greyhawk gods, would be fantastic. Essentially the recent Dragon magazine gods of greyhawk series in book form. I'd also work hand in hand with Mr. Nard to create evocative and useful spells, feats, and talents (or whatever distinguishes between different types of clerics in 4e) for each of the priesthoods.
A new planescale setting book. Don't just focus on Sigil - reintroduce the entire planescape setting for 4th edition. Open it up a bit - don't just focus on the great wheel. Create a greater emphasis on demiplanes and planar pockets, so DMs can introduce any planar content they want. A book with the size and level of detail of the 3e Forgotten Realms book, devoted to the planescape setting, and planar adventuring in general. Use a combination of old-style planescape art with evocative new art.